1. Field of the Invention
The present invention, the Moisture and Density Detector (MDD), relates to an apparatus and method for detecting the moisture content (MC) and/or density of dielectric materials.
2. Related Art
Moisture Estimation Using Radio Frequency Signals
Several devices have been developed to measure moisture in materials. These devices are based primarily on resistance and capacitance principles. Resistance is the opposition of a body or substance to a current passing through it. Capacitance is the property of a circuit element that permits it to store charge. For resistance devices, a direct current (DC) radio frequency signal is passed through a dielectric material (a material that does not conduct electricity) and the signal strength is measured as a function of the resistance of the material. This resistance measurement is then converted to a moisture content (MC) value using correction factors for temperature and species. Capacitance devices measure capacitance value or xe2x80x9cpower-lossxe2x80x9d and estimate MC based on known correlation.
U.S. Pat. No. 4,259,633 to Rosenau describes a resistance MC estimation technique. The technique applied by Rosenau and others is limited in that it requires that metal pins be inserted into the wood sample being tested. In addition, the electrolytic polarization effects when using DC voltage can result in measurement error. Inserted-pin resistance devices are considered to provide inaccurate estimates when the wood MC is above the fiber saturation point of 24 to 30 percent.
U.S. Pat. No. 3,600,676 to Lugwig et al. teaches the capacitance technique whereby an alternating current (AC) radio frequency capacitance device was developed using adjacent electrodes and resonance to determine the MC of bulk materials (i.e., coal, chips, etc.). This device applies a range of frequencies to the dielectric material adjacent to the electrodes. The frequency with maximum signal strength is termed the resonant frequency and is a direct function of the MC of the dielectric material. The Lugwig et al. device determines the resonant frequency at which signal strength (amplitude) reaches a maximum. Applicant""s invention also applies a range of frequencies to the dielectric material and measures the signal strength of each in terms of amplitude. However, Applicant""s invention does not determine the resonant frequency but rather relates the measured amplitude of each frequency to predetermined values to determine the MC of the dielectric material. In addition, in contrast to Applicant""s invention, the Lugwig et al. device does not use phase shift as additional information to estimate MC or density.
U.S. Pat. No. 4,616,425 to Burns describes an opposed electrode device based on resistance or capacitance controlled oscillator circuits. Whether based on resistance or capacitance, this device requires conversion to a frequency-dependent DC voltage. Signal strength of the DC voltage is related to predetermined voltage values for the dielectric material to allow MC estimation. Direct contact with the dielectric material is required. In contrast to Burns, Applicant""s invention does not employ conversion from AC signal to DC signal. In addition, direct physical contact with the wood surface is possible, but not necessary. Furthermore, Burns does not use measurement of phase shift to improve the MC estimate. The Burns device also has no capability to estimate dielectric material density.
U.S. Pat. No. 3,430,357 to Perry discloses an opposed electrode device that measures capacitive impedance and associated MC in a stack of lumber in a dry kiln. The resistance between a capacitance probe inserted several courses of lumber above a ground electrode gives a measure of stack MC in the lumber between the electrodes. This method requires direct contact between the capacitance probe and the lumber. With the Perry device, an AC signal is converted to a DC signal prior to measurement of the signal strength as voltage. Perry differs from Applicant""s invention in that Applicant directly measures the strength of the AC signal. Perry also does not employ a phase shift measurement to improve the MC estimate. In addition, the Perry device has no capability to estimate dielectric material density.
U.S. Pat. No. 4,580,233 to Parker et al. teaches an adjacent electrode AC moisture sensing device with two alternating frequencies that measures the imbalance in a capacitance bridge to estimate the MC of dielectric materials. Circuitry and methodology is incorporated to correct for potential wood temperature differences. As with the Lugwig et al., Burns, and Perry disclosures, the AC signal is converted to a DC signal prior to measurement of voltage to determine signal strength. This differs from Applicant""s invention, which directly measures the strength of the AC signal. In addition, Parker et al. does not employ phase shift either to improve the MC estimate or to allow for estimation of dielectric material density.
U.S. Pat. No. 5,402,076 to Havener et al. recites a portable device, similar to Perry""s device, that measures MC in a stack of lumber but with the AC radio frequency signal transmitted between adjacent electrodes. As with Perry, Applicant""s invention differs because Applicant measures the phase shift and has the capability to estimate wood density.
U.S. Pat. No. 5,486,815 to Wagner discloses an in-line AC moisture meter employing opposed capacitance electrodes to sense MC in lumber moving between the electrodes. A single 4 MHz frequency is transmitted between electrodes and the received signal strength is measured to provide an estimate of the wood MC. The 4 MHz signal is applied to two pairs of electrodes with a 20-volt peak-to-peak amplitude signal applied to one pair and a 4.5 volt peak-to-peak amplitude signed to the other. The 4.5 volt signal is applied 180xc2x0 out-of-phase with the higher 20-volt signal. Wagner teaches that analysis of the out-of-phase signal responses reduces the effects on the signal of electrical loading of the material. Wagner differs from Applicant""s invention because Wagner does not improve the estimate of MC by adding phase shift information and Wagner has no described capability to estimate the dielectric material density. This device is also limited to detection of MC below 24 percent.
The teachings described above have employed measures of signal strength of both resistance and capacitance electrodes to estimate dielectric material MC. Both AC and DC devices have been developed. However, none of the described devices are reportedly accurate in measuring MC above the fiber saturation point of approximately 24 to 30 percent MC. In addition, none have employed measurement of signal phase shift to improve their estimate of MC. Furthermore, none report the capability of estimating the density of the dielectric material by combined analysis of amplitude and phase shift of a radio frequency signal.
U.S. Pat. No. 5,086,279 to Wochnowski discloses a means for estimating MC in a stream of materials by both reflecting and passing electrical energy through the stream in the form of infrared, microwave, or energy generated by a high-frequency oscillator circuit. For each of the electrical energy types, the energy is both reflected from and transmitted through the material stream. The transmitted energy from the high-frequency oscillator may be inferred to be in the same radio frequency range as Applicant""s invention, although Wochnowski did not define the spectrum.
The Wochnowski MC estimate of the stream of materials depends on measures of signal strength and phase shift with each obtained by two methods. The two methods are to obtain a reflected signal detected by a sensor on the same side of the stream of materials and also a through signal such as is obtained by an opposed or adjacent electrode configuration. Therefore, the MC estimate provided by Wochnowski depends partially on the correction for the mass of the stream of materials by analysis of the xe2x80x9cdamping of oscillationsxe2x80x9d of electromagnetic waves through a first signal and a second (reflected) signal. Likewise, additional information for the MC analysis is obtained from the phase shift of both a through and reflected signal.
Applicant""s invention differs from Wochnowski in that it requires no information on reflected energy but depends solely on its estimate of MC and density based on passage of the signal between the electrodes. In addition, Applicant compares phase shift and signal strength changes, caused by interaction of the radio frequency signal with the dielectric material, to predetermined values to provide the estimation of MC and density. Wochnowski describes no method for comparing predetermined values to correlate measured phase shift and signal strength decrease to expected values for the dielectric material at given MC""s and densities. Applicant also provides an estimate of dielectric material density that the Wochnowski device does not provide.
In a 1993 writing, Torgovnikov discloses dielectric constants, measures of signal strength, and loss tangent values for radio frequencies from 20 to 1000 Hz. G. Torgovnikov, DIELECTRIC PROPERTIES OF WOOD AND WOOD-BASED MATERIALS 174-181 (1993). (The terms loss tangent and phase shift are both referenced herein. While these terms differ in their meaning, they are mutually direct functions with one easily derived from the other. For this reason, devices designed to provide information for one also indirectly provide the other value. In that sense these terms will be used interchangeably.) For all frequencies tested, Torgovnikov shows via plotted regressions that the rate of increase in the dielectric constant is higher for MC below the fiber saturation point. The plotted slopes of the regression lines also appear to have significant slope above the fiber saturation point. These plotted regression lines, however, represent the mean dielectric response for a range of wood specific gravity values.
Torgovnikov also teaches that the dielectric response is strongly influenced by the wood specific gravity. Therefore, dielectric constant information alone will not allow an accurate estimation of wood MC because of the confounding influence of wood density. With current methods this confounding influence can only be eliminated if a single wood density is scanned or if the density of specimens is known. Torgovnikov does not provides a method to improve MC estimate by including phase shift or loss tangent as a predictive variable.
Moisture Estimation Using Microwaves
Attempts have been made to measure the MC of materials using microwave energy. U.S. Pat. Nos. 4,727,311 and 5,767,685 to Walker teach ways to measure the MC of materials such as sand and coal. In these cases, two microwave frequencies are passed through a material in order to determine MC. The difference between the two signals assists in determining MC.
U.S. Pat. No. 4,674,325 to Kiyobe et al. calculates MC by passing material between non-contacting microwave horns. The basis weight is detected with an ionizing chamber.
U.S. Pat. No. 5,315,258 to Jakkula et al. discloses a radar system developed for measuring the MC of materials. There, the change in velocity of the microwaves within the material is correlated to differences in MC.
The Walker, Kiyobe et al., and Jakkula et al. teachings differ from Applicant""s invention in that a microwave signal rather than signals in the radio frequency spectrum are utilized. Microwave devices require wave guides to transmit and receive the signals while radio frequency devices such as the Applicant""s require only electrodes. These microwave devices described also do not have the capability to estimate dielectric material density.
The following disclosures describe microwave devices based on the attenuation of the microwave signal to estimate moisture content combined with information on phase shift of the microwave signal to provide wood density information:
1. R. King et al., Microwave Measurement of the Complex Dielectric Tensor of Anisotropic Slab Materials, in PROCEEDINGS OF A TECHNOLOGY AWARENESS SEMINAR (Nov. 15-16, 1987).
2. R. King et al., Measurement of Basis Weight and Moisture Content of Composite Boards Using Microwaves, in PROCEEDINGS OF THE 8TH SYMPOSIUM ON THE NONDESTRUCTIVE TESTING OF WOOD (Sep. 23-25, 1991).
3. P. Martin et al., Evaluation of Wood Characteristics: Internal Scanning of the Material by Microwaves, in 21 WOOD SCIENCE TECH. 367-371 (1987).
4. P. Martin et al., Industrial Microwave Sensors for Evaluation of Wood Quality, in FOURTH INT""L CONFERENCE ON SCANNING TECHNOLOGY IN THE WOOD INDUSTRY (1991).
5. J. Portala and J. Ciccotelli, Nondestructive Testing Techniques Applied to 
Wood Scanning, in 2 INDUSTRIAL METROLOGY 299-307 (1992).
King et al. (1987), King et al. (1983), Martin et al. (1987), Martin et al. (1991), and Portala et al. (1992) depend for their estimates of MC and density on the analysis of both attenuation and phase shift. Microwave devices differ from Applicant""s device in that microwave frequencies are in the range above 1000 MHz. For frequencies in this range waveguides that are much more costly than the electrodes of Applicant""s device are required. No radio frequency device has been disclosed that combines analysis of changes in signal amplitude and phase shift to estimate wood MC and density, with the exception of Wochnowski. As discussed, this device requires information on both reflected and through-material amplitude and phase shift signals to obtain estimated material MC and density.
Radio Frequency Moisture Gradient Estimation
An impedance detector disclosed by Tiitta et al. measures the moisture gradient in wood. M. Tiitta et al., Development of an Electrical Impedance Spectrometer for the Analysis of Wood Transverse Moisture Gradient, in PROCEEDINGS OF THE 12th INT""L SYMPOSIUM ON NONDESTRUCTIVE TESTING OF WOOD (Sep. 13-15, 2000). Electrodes contained in a probe are placed on the wood surface. One electrode transmits an electrical signal at frequencies below 5 MHz, and the second receives the signal. A variable electric field is developed between the electrodes. Analysis of the behavior of impedance, or signal strength, for the various frequencies transmitted through the wood allows estimation of the moisture gradient within the wood. This device was developed to sense the moisture gradient in logs.
Writings by Sobue and Jazayeri et al. have demonstrated a method to sense the moisture gradient in wood by what Sobue termed Electrode Scanning Moisture Analysis (ESMA). N. Sobue, Measurement of Moisture Gradient in Wood by Electrode Scanning Moisture Analysis ESMA, in PROCEEDINGS OF THE 12th INT""L SYMPOSIUM ON NONDESTRUCTIVE TESTING OF WOOD (Sep. 13-15, 2000); S. Jazayeri and K. Ahmet, Detection of Transverse Moisture Gradients in Timber by Measurements of Capacitance Using a Multiple-Electrode Arrangement, 50 FOREST PROD. J. 27-32 (2000). ESMA determines MC at various depths through wood thickness by manipulating the distance between adjacent electrodes on a single wood surface between 0.43 in. (11 mm) and 1.97 in (50 mm), shown in FIG. 1. Examination of the capacitance changes developed by manipulation of electrode distance allows computation of wood moisture gradient at various depths through wood thickness. Sobue""s method allowed measurement of MC in wood up to 120 percent. Sobue and Jazayeri et al., however, demonstrated that this method would work for only a single wood density in which MC levels were manipulated.
The Tiitta et al., Sobue, and Jazayeri et al. devices are adjacent electrode impedance devices that are designed to estimate moisture gradient rather than average MC. The ability to estimate wood density as well as moisture gradient was not demonstrated by this device. By contrast, Applicant""s invention is an opposed or adjacent plate capacitance device that senses mean MC and may also provide an estimate of wood density. Neither the Tiitta et al., Sobue, or Jazayeri et al. devices employ phase shift to improve their estimate of MC or to provide an estimate of wood density.
U.S. Pat. No. 5,585,732 to Steele et al. and two writings by Steele et al. have disclosed a method for detecting density differences in scanned lumber by a radio frequency method with opposed electrodes. P. Steele and J. Cooper, Estimating Strength Properties of lumber with Radio Frequency Scanning, in PROC. OF THE 4TH INT""L CONFERENCE ON IMAGE PROCESSING AND SCANNING OF WOOD (Aug. 21-23, 2000); P. Steele et al., Differentiation of Knots, Distorted Grain, and Clear Wood by Radio-Frequency Scanning, 50 FOREST PROD. J. 58-62 (2000). To date, only detection of knots and voids has been described as being detected. The application of phase shift or loss tangent to assist in more accurately estimating dielectric material MC or estimating density has not been disclosed for this or any other radio frequency device.
The disclosures by Steele et al. employed dielectric properties and wood density in the estimation of wood strength by radio frequency capacitance employing a variation of the Steele et al. device. However, the Steele et al. method depended on prior knowledge of wood MC with statistical correction for the known MC differences. Validation of this method showed an R2 value of 0.67 between attenuated dielectric signal and lumber modulus of rupture. Only a single radio frequency signal attenuation measurement to provide specific gravity estimates was employed. Applicant""s invention, by contrast, may employ single or multiple radio frequency signals to obtain dielectric constant. The Steele et al. method did not measure phase shift to improve the estimate of wood density.
Wood Strength Estimation Based on Density Detection
The amount of lumber graded by machine stress rating (MSR) has continued to increase since the development of the technology in the early 1960""s. This growth has been driven by the significant premium in value for MSR versus visually graded lumber in certain lumber grades. MSR graded lumber is mechanically flexed to obtain a flatwise modulus of elasticity. In a 1968 writing, Muller teaches a method of estimating lumber grade based on the known relationship between modulus of elasticity and modulus of rupture combined with additional information from visual inspection of the lumber. P. Muller, Mechanical Stress-Grading of Structural Timber in Europe, North America and Australia, 2 WOOD SCI. and TECH. 43-72 (1968). In addition, in 1997 Biernacki et al indicated a significant potential for increased lumber value from improved accuracy in lumber grading. R2 values based on relating modulus of elasticity to modulus of rupture are species dependent but are approximately 0.50. J. Biernacki et al., Economic Feasibility of Improved Strength and Stiffness Prediction of MEL and MSR Lumber, 47 FOREST PROD. J. 85-91 (1997).
U.S. Pat. No. 4,941,357 to Schajer discloses an alternative to MSR lumber grading that is a system that estimates lumber strength based on x-ray through-lumber-thickness scanning. By this method the lumber strength is estimated by assigning a clear wood strength value with deductions based on knot presence indicated by specific gravity scans. Lumber strength estimations based on x-ray scanning is reported to be higher than MSR estimates with R2 values ranging between 0.68 and 0.78 for southern yellow pine lumber.
Applicant""s invention has potential as an MSR device capable of predicting clear wood density. In such use, Applicant""s invention will require a knot detection system such as a digital camera, ultrasound, radio frequency, infrared, etc. MSR lumber grading requires information on knot size and location in addition to density of clear wood. Also required will be techniques and software to correct for knot influence on lumber strength.
After logs are milled and lumber is created, the lumber is usually dried. Softwood lumber is a challenge to dry, and hardwood lumber is even more difficult. A key difference between hardwood and softwood lumber drying is the initial moisture content (MC) at the start of kiln drying. Wood MC may vary from 0% to a xe2x80x9cgreenxe2x80x9d measure. A green measure of MC may be as high or higher than 200 percent. Softwood lumber is dried green immediately after it is sawn. The average initial MC of softwood lumber is often greater than one-hundred percent, based on oven-dry weight. Typically, hardwood lumber has a significantly lower initial MC than softwood lumber. When dried directly from the saw, hardwood lumber is typically between sixty and eighty percent MC while softwood lumber frequently exceeds 100 percent MC. Often, hardwood lumber is air dried to reduce its initial MC to approximately 25 percent before being dried. In contrast, softwood lumber is often dried green at, or above, 100 percent MC.
Lumber in dry kilns can be monitored for drying rate and for final moisture content (MC) at the drying schedule endpoint by either MC schedules or time-based schedules. MC schedules monitor the rate of drying by periodically weighing previously cut short lumber samples during drying to measure the MC. Time-based schedules do not require lumber samples, but instead assume that the rate of drying is correlated with kiln conditions and the time over which the conditions are applied to the lumber. Time-based schedules are widely used in the drying of softwood lumber because softwood lumber is less susceptible to drying degrade caused by drying the wood at an improper rate. However, failure to control the drying rate when applying pre-set time schedules is responsible for considerable lumber degrade during drying. In order for time-based schedules to work well, each lumber load placed in the kiln must have approximately the same initial MC, the same permeability, and kiln conditions must be identical from charge to charge. These requirements are not always satisfied and lumber drying degrade often occurs.
For most hardwood species, MC schedules must be used to prevent dramatic value losses from drying degrade. Traditional MC schedules require the kiln operator to control the drying rate by monitoring the MC of several kiln samples. These samples are two to three-feet long and are dried with the kiln charge of lumber. Prior to the start of drying, a MC section is cut from each kiln sample. This section is rapidly oven dried to determine the initial MC of the wood going into the kiln. This initial MC value is used in conjunction with the sample weight to determine the samples"" MC throughout the drying run. This continual monitoring allows control of the kiln conditions, and the lumber""s drying rate is based on the average sample MC.
The process of monitoring kiln samples requires kiln operators to repeatedly enter the dry kiln to remove kiln samples for monitoring by weighing. Following weighing, the kiln samples must be returned immediately to the dry kiln. Softwood lumber of nominal two-inch thickness is dried at high temperatures from green wood and the drying process usually requires less than 24 hours. In the case of air-dried hardwoods of four-quarter-inch thickness, the approximate drying time is between four and eleven days.
Monitoring MC samples over a short-time interval (24 hours or less) makes it difficult for operators to apply MC schedules for softwoods without additional technology. For both hardwood and softwood, kiln drying technological developments in recent years have produced several new methods to estimate the MC drying rate and drying end point. Reports on the effectiveness of these systems has been provided by Culpepper. L. Culpepper, HIGH TEMPERATURE DRYING 258-262 (1990).
One method for estimating the MC drying rate is temperature drop-across-the-load monitoring, which monitors the temperature of the air flowing across the drying lumber. The air temperature decrease from lumber entry to lumber exit is closely correlated to the wood MC for MC below the fiber saturation point, but the method is inaccurate above the fiber saturation point.
Another method for estimating the MC drying rate uses electrical resistance devices and employs pairs of pins inserted into holes drilled into the lumber. The distance between the pins is limited (1xe2x80x3 to 2xe2x80x3) to allow an applied low voltage to flow between the two pins. The resulting resistance is measured and correlated to wood MC. The resistance devices accurately predict the MC below the fiber saturation point, but inaccurately predict the wood MC above the fiber saturation point.
An additional method uses an electrical capacitance method to measure the capacitance between plates inserted in the stack and kiln rails which are grounded. This method has been shown to provide a MC measurement that is often not accurate.
Weight-based systems are another method used to measure wood MC. These systems measure the total weight of kiln lumber during drying. This allows close monitoring of the drying rate. These systems are reportedly accurate, but problems with sensor durability in the harsh kiln environment and the relatively high cost of using weight-based systems has limited their widespread adoption.
A more recent weight-based system not reviewed by Culpepper monitors the weight of a kiln sample suspended in the kiln plenum, which is the space around the lumber. This system is reportedly effective but the relatively high cost of the system has limited its adoption.
As summarized above, systems that measure MC during kiln drying to monitor drying rate and drying end point are available. However, these systems are relatively expensive and not always effective at monitoring wood MC. There exists a need for improved methods to monitor drying rate and drying end point.
The ability to estimate wood density is also needed in the wood processing industry. The term density as employed herein refers to the oven-dry specific gravity of a dielectric material. A dielectric material is a material that does not conduct electricity. For hygroscopic materials that readily absorb water, such as wood, the specific gravity differs from the density when the material MC is greater than zero. Wood density varies significantly, both between and within species. In addition to the need to detect the MC and/or density of lumber, there exists a need to detect the MC and/or density of wood particles and flakes, wood composite products, and other dielectric materials (such as rubber, plastics, and foods).
The present invention, a Moisture and Density Detector (MDD), can provide a method and apparatus to determine the MC and/or density of any dielectric material for various purposes. Examples of dielectric materials that can be used with the MDD include, but are not limited to, wood-based materials, wood composites, agricultural and food products, plastic, and rubber.
The present invention can sense the dielectric response of a radio frequency signal passed between opposed or adjacent capacitance electrodes and can measure the signal strength and phase shift of the signal. The addition of phase shift and multiple frequencies can improve the accuracy of the results of this type of device for multiple layer scanning.
Although the MDD can be used for any dielectric material, it is very useful in detecting the moisture content (MC) of wood and wood-based materials. In particular, the MDD can be used to detect the MC and/or density of lumber in a dry kiln prior to, during, and/or following drying. In addition, the MDD can detect the MC and/or density for the purpose of assigning lumber or veneer strength grades. The MDD can also detect MC and/or density of lumber, logs, poles, flakes, particles, composite panels, or any other form of solid wood product for any other purpose.
The MDD can also be used to monitor green wood MC prior to and following drying. In this case lumber, veneer, flakes, particles, etc. can be monitored and subsequently sorted on the basis of MC. Green sorting of lumber by weight is commonly done at sawmills that wish to maximize kiln capacity. Lumber with different MCs can be dried separately allowing lumber with lower MC to be dried more rapidly. Dry sorting to detect wet wood both between and within individual pieces of lumber to identify those pieces requiring further drying is also a potential application.
In addition, the MDD can be used as a machine stress rating (MSR) device by which lumber strength is assessed based on density. A MSR device is commercially available that employs x-rays for this purpose, but no radio frequency device is available.
The above and other objects and advantages of the present invention will become more apparent from a reading of the following detailed description of the invention in conjunction with the drawings and appended claims.